Multiple solid state RF electronic devices, such as integrated circuits, that provide individual circuit functions at RF frequencies, such as millimeter microwave frequencies, are often housed together in a single closed metal-walled package or, as variously termed, multi-chip module. The package or module is typically hermetically sealed and protects the confined electronic devices from the external environment, which sometimes contains radiation, corrosive gases or other material harmful to the confined devices. To carry RF signals through the package's metal wall between the module's interior and exterior, RF feed-throughs that are integral with the module are employed. In essence, the feed-through is a very short RF transmission line. It is the means conventionally used to propagate RF energy through an RF barrier, such as a metal wall. The feed-through typically connects to RF connectors or radiates into waveguides attached to the package exterior.
Typically feed-throughs have been constructed of glass and metal. The glass, referred to as a glass bead, located in a hole in a package wall, serves as an insulating support and dielectric that maintains a straight metal pin, the transmission line conductor, in electrically insulated relationship with the package walls and serves as an impervious barrier to the external environment. A single glass bead may support multiple pins.
Despite its effectiveness, the glass-to-metal seals suffer an important drawback: they are not durable. The glass is brittle. If the feed-through's glass encased metal pin is deflected, bent or deformed during handling or testing, glass particles are broken at the glass meniscus surrounding the pin. That breakage compromises the integrity of the feed-through. In some cases, radial cracks or circumferential cracks appear in the glass. Those cracks might be due to differences in thermal expansion characteristics between the glass and the pin, or some form of fatigue or from other causes, which remain unknown.
However, once even a small crack appears, the crack may propagate with repeated thermal cycling as occurs during normal use of the electronic apparatus containing the package. Once crack propagation occurs, mechanical movement of the package or mechanical stresses resulting from handling, shipping, aircraft or spacecraft vibration may aggravate the cracks and the feed-through begins to noticeably leak. Being aware of the glass's fragility, those skilled in fabricating devices containing those RF feed-throughs necessarily take extra care in handling to ensure the integrity of the product, which drags down manufacturing efficiency. As an advantage, the present invention eliminates all glass to metal feed throughs from the multi-chip module package.
In those prior multi-chip modules, it was not practicable to assemble all of the integrated circuit chips of the different RF circuits upon a single substrate, an indirect consequence in part of the use of glass to metal feed throughs. The multiple RF circuits within a chip module were fabricated on separate substrates. The metal housing base was machined to carve out from a metal block the separate RF isolating compartments, accessible from the top of the base. These separate substrates were separately placed into the respective compartments within the metal housing and soldered in place. The metal base was both expensive to produce and larger in size than desired.
As a further advantage, the present invention permits fabrication of all of the RF circuits upon a single substrate, increasing manufacturing efficiency, reliability and, importantly, decreasing fabrication costs. The invention eliminates the need to carve out separate RF compartments within the metal base, further reducing fabrication cost.
Accordingly, an object of the present invention is the elimination of glass feed throughs in multi-chip millimeter wave integrated circuit packages.
A further object of the invention is to reduce the physical size and cost of millimeter microwave multi-chip modules.
And a still further object of the invention is to enhance the reliability and manufacturing efficiency of multi-chip modules.
The digital semiconductor chip packaging art is known to employ an array of pins, referred to as a pin grid array, to carry the power supply voltages and communicate digital data between the semiconductor chip and other apparatus exterior to that chip. In that field the pin grid array offers, among other things, the opportunity to increase the density of electrical contacts and/or permit greater miniaturization of the chip assembly. Accordingly, an additional object of the invention is to adapt such pin grid away technique to the construction of millimeter wave multi-chip modules and to provide thereby a new RF pin grid array.
In preparing the present record of the invention, applicant was made aware of a U.S. patent to Nicholson, issued Sep. 16, 1997, filed Apr. 12, 1996, entitled "Pin Grid Array Solution for Microwave Multi-Chip Modules". Nicholson offers the concept of having a pin grid array type package as a means to increase the density of external connections to the module and offers a package construction that incorporates both depending pins in the substrate for DC and RF transmission and adhesive between the package lid and substrate and between the substrate and base that are intended to address differential expansion problems between the package and due to the sideways flexure allowed by the pins.
In that structure the pins are of identical size, 0.020 inches in diameter with a slightly larger 0.028 inch diameter mounting pedestal for interface with a SMA connector. The pins extend from the top surface of the substrate where anchored, through the thickness of the substrate, out the bottom end of the substrate, and through corresponding pin passages through the metal base from which the pins protrude. Nicholson notes that in extending through passages in the metal base the pins and the surrounding metal passage wall form coaxial airline structures.
The completed package is then bolted to a PC board with a conductive gasket in between and the extending pins are then soldered to the backside of the PC board at the locations at which they protrude through. And a metal backplate that is threaded to accept SMA connectors is then bolted to the backside of the PC board, also with a gasket in between.
Although Nicholson's technique for adapting pin grid array structure to a multi-chip module is interesting, it employs adhesives and thus does not include any capability for hermeticity, a characteristic known to provide the greatest reliability and operational life. Moreover, to accommodate integrated circuits of higher power Nicholson must cut away the supporting substrate and rest the integrated circuit on the metal plate underlying the substrate in order to provide a sufficient thermally conductive path to a heat sink and avoid thermal damage to the integrated circuit. It may also be noted that Nicholson does not incorporate a waveguide within the multi-chip module or a microstrip to waveguide transition, an ancillary feature of the present invention. As becomes apparent from the description which follows, the present invention offers an alternative solution, one that permits a package of simpler structure and of a more compact physical size, hermeticity and waveguide transitions.
Accordingly, an additional object of the invention is to provide RF feed-throughs through waveguide transitions in a multi-chip module for coupling RF to a waveguide.
It is a still additional object of the invention to ensure hermeticity in the feed-through for greatest operational life.
And it is an ancillary object of the invention to provide a new multi-chip module that eliminates the need to include cut out sections in the substrate in which to mount integrated circuits and permits heat to conduct from the integrated circuits through the substrate in all instances.